1. Field of the Invention
The present invention relates to an LED lamp including a wavelength converting portion with a phosphor.
2. Description of the Related Art
White LED lamps are recently under vigorous research and development as potential replacements for white incandescent lamps. In some of those white LED lamps, the package of a blue LED chip, made of gallium nitride (GaN), is coated with a phosphor such as YAG. In such an LED lamp, the blue LED chip produces an emission with a wavelength of about 450 nm, and the phosphor produces yellow fluorescence with a peak wavelength of about 550 nm on receiving that emission. Eventually, the emission and fluorescence mix with each other, thereby providing white light.
In another type of white LED lamp currently under development, an LED chip that emits an ultraviolet ray is combined with a phosphor that produces red (R), green (G) and blue (B) light rays. In such an LED lamp, the ultraviolet ray that has been radiated from the LED chip excites the phosphor, thereby emitting the red, green and blue light rays. Consequently, white light can also be obtained as a mixture of these light rays.
A bullet-shaped package is extensively used in conventional LED lamps. Hereinafter, such an LED lamp with a bullet-shaped appearance will be described with reference to FIG. 1.
FIG. 1 illustrates a cross-sectional structure for a conventional LED lamp 20 as disclosed in Japanese Patent No. 2998696, for example. As shown in FIG. 1, the LED lamp 20 includes an LED chip 21, a bullet-shaped transparent housing to cover the LED chip 21, and leads 22a and 22b to supply current to the LED chip 21. A cup reflector 23 for reflecting the emission of the LED chip 21 in the direction indicated by the arrow D is provided for the mount portion of the lead 22b. The inner walls (i.e., reflective surfaces) of the cup reflector 23 surround the side surfaces of the LED chip 21 so as to define a predetermined tilt angle with respect to the bottom of the cup reflector 23. The LED chip 21 on the mount portion is encapsulated with a first resin portion 24, which is further encapsulated with a second resin portion 25.
The first resin portion 24 is obtained by filling the cup reflector 23 with a resin material and curing it after the LED chip 21 has been mounted onto the bottom of the cup reflector 23 and then has had its cathode and anode electrodes electrically connected to the leads 22a and 22b by way of wires. A phosphor 26 is dispersed in the first resin portion 24 so as to be excited with the light. A that has been emitted from the LED chip 21. The excited phosphor 26 produces fluorescence (which will be referred to herein as “light B”) that has a longer wavelength than the light A. This LED lamp 20 is designed such that if the light A radiated from the LED chip 21 is for example, red, then the light B emitted from the phosphor 26 is yellow. A portion of the light A is transmitted through the first resin portion 24 including the phosphor 26. As a result, light C as a mixture of the light A and light B is used as illumination light. The light A may also, for example, exhibit a narrow-band spectral distribution with a peak wavelength of about 470 nm, while the light B may exhibit a broad-band spectral distribution with a peak wavelength of about 570 nm, for example.
The conventional LED lamp shown in FIG. 1, however, does not have good color rendering properties. FIG. 2 shows an exemplary spectral distribution of a white LED lamp having the configuration shown in FIG. 1. As can be seen from FIG. 2, the quantity of light emitted from such a white LED lamp in the red wavelength range is significantly lower than in the other wavelength ranges. Thus, such a white LED lamp exhibits deteriorated color rendering properties in that red wavelength range.
The “color rendering properties” is a general expression for the effect of an illuminant on the color appearance of an object that is illuminated by the illuminant. The color rendering properties of an illuminant may be evaluated by its average color rendering index Ra. That is to say, the higher the Ra index of an illuminant, the better the color rendering properties thereof.
According to ISO 8895:1989, illuminants with average color rendering indices Ra of 90 or more are classified as Group 1A, while illuminants with average color rendering indices Ra of 80 to less than 90 are classified as Group 1B. A preferred average color rendering index Ra of an illuminant is changeable with the specific application of the illuminant but is normally at least 70.
However, depending on the color of the light to be provided, it is often hard for the conventional white LED lamp including a blue LED chip to achieve an average color rendering index Ra of 70 or more.